It has long been known to separate air in an air separation plant that employs cryogenic rectification to separate air into its component parts to produce oxygen, nitrogen and argon products. Typically, the air is compressed and purified of the higher boiling contaminants and is then cooled within a main heat exchanger to a temperature suitable for its rectification within a distillation column.
Air separation plants that are designed to produce oxygen and nitrogen products have high and low pressure columns that are operatively associated with one another in a heat transfer relationship by a condenser reboiler. The incoming compressed and cooled air is introduced into the bottom of the high pressure column to separate the air into a nitrogen-rich vapor column overhead and an oxygen-rich liquid column bottoms also known in the art as crude liquid oxygen or kettle liquid. A stream of the oxygen-rich column bottoms is introduced into the low pressure column in order to separate the stream into an oxygen-rich liquid and a nitrogen-rich vapor, both of which can be taken as the oxygen and nitrogen products, respectively. If argon is a desired product, an argon column can be connected to the low pressure column to separate the argon from an argon-rich stream removed from the low pressure column.
Reflux to both the high and low pressure column is effectuated by condensing the nitrogen-rich vapor developed as column overhead in the high pressure column. This condensation occurs within a condenser reboiler in which an oxygen-rich liquid partially vaporizes against condensing the nitrogen-rich vapor.
There are different designs that are used in the fabrication of such condenser reboilers. In one common design, known as a thermosiphon reboiler, the oxygen-rich liquid collected as column bottoms in the low pressure column partially vaporizes into a two-phase liquid-vapor mixture that is discharged from the reboiler outlet. The density difference provides sufficient liquid head to deliver the column bottoms to the reboiler. There are also down-flow types of reboilers for instance, formed by a brazed aluminum heat exchanger that is fed with the oxygen-rich liquid from a liquid reservoir located at the top of the reboiler. As the oxygen-rich liquid descends within the passages of such a reboiler, it partially vaporizes into liquid and vapor fractions of phases. The passages within such a heat exchanger are formed by parallel sheets of aluminum that are brazed together that have internal fins to increase the heat transfer area. In addition there are reboilers of the down-flow shell and tube design in which tubes are attached to upper and lower tube sheets. A reservoir is located above to distribute the oxygen-rich liquid to the tubes. Inlets and outlets are provided in the shell for the nitrogen-rich vapor to enter the shell and condense through indirect heat exchange with the oxygen-rich liquid. Vapor and liquid phases of the oxygen-rich liquid are discharged from the bottom of the tubes. Down-flow condenser reboilers of both the plate-fin design and the shell and tube design are advantageous in that they permit closer approach temperatures of the liquid and vapor to be brought into indirect heat exchange. This results in a lower compression requirement for the incoming air.
In order for a down-flow condenser reboiler to function, the partially vaporized liquid must be able to be discharged from the bottom of the passages or tubes. However, during a cold plant shut-down, the oxygen-rich liquid present within the lower pressure column will dump into the sump provided at the bottom of the low pressure column resulting in the partial submergence of the down-flow condenser reboiler. While liquid could be dumped to clear the passages or tubes, this represents a loss in refrigeration and in addition, in order to restart the plant, additional liquid might have to be brought in by truck or rail for such purposes.
The problem of restarting an air separation plant after a cold shut-down when down-flow condenser reboilers are used has been addressed in U.S. Pat. No. 5,071,458 wherein an auxiliary condenser reboiler is provided that is located above the main condenser reboiler at a sufficient height in the column that upon plant shut-down, it will not be submerged and the plant can be restarted. The problem with such an arrangement is that the column must be taller to accommodate the auxiliary condenser reboiler. This results in undesirable increased fabrication costs as well as potential problems in shipping longer than average columns.
As will be discussed the present invention provides a condenser reboiler system of the down-flow type that is used in connection with an air separation plant in which the column can be restarted without draining any liquid from the sump and that has as an advantage, among others that will become more apparent hereinafter, a deployment of down-flow condenser reboilers that does not require the column to be enlarged to accommodate an auxiliary condenser reboiler.